slirp: Remove superfluous memset() calls from the TFTP code

slirp: Remove superfluous memset() calls from the TFTP code

Commit fad7fb9ccd8013ea03  ("Add IPv6 support to the TFTP code")
refactored some common code for preparing the mbuf into a new
function called tftp_prep_mbuf_data(). One part of this common
code is to do a "memset(m->m_data, 0, m->m_size);" for the related
buffer first. However, at two spots, the memset() was not removed
from the calling function, so it currently done twice in these code
paths. Thus let's delete these superfluous memsets in the calling
functions now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>